Automatic volume control circuit



June S, 1946.

W. J. POCH AUTOMATIC VOLUME CONTROL-CIRCUIT Filed May 15, 194s @.mf. kmos mugs. ES

A INVENTOR. Waldemar l P/oh/ A fro/e /EY Patente .Pune i8, v1946 AUTOMATIC VOLUME CONTROL CIRCUIT Waldemar J. Pooh, Moorestown, N. J., asslgnor to; Radio Corporation of America, a corporation of Delaware Application May 13,1943, Serial No.' 486,789 s claims. (ci. 17a-:1.2)

The present invention is related to television circuits and particularly to television ampliiier circuits, and is mainly concerned with automatically setting or controlling Athe video output signal level in such amplifier circuits.

The circuit Vto be described is intended primarily for use with television apparatus and systems wherein the video or picture signals representing the scene being scanned result from the scanning of a storage element. such as the mosaic element of cathode ray scanning or camera tubes of the general types known in the art as the Iconoscope," the Image Iconoscope and the "Orthicon" for instance, all oi which have been generally described by pages 94 etrseq. of the book Principles of Television Engineering, by D. G. Fink, and published in 1940 by McGraw-Hill Book Co., Inc., of New York.

In such systems, an optical image of the scene to be televised is cast upon the mosaic velectrode of the scanning tube and produces thereon electrostatic charges of magnitudes proportional to the image brightness. When the charges so produced are4 scanned by a, cathode ray scanning beam, output signals, known as the video or pic-.

ture signals, result and are released to a signal utilization circuit, which usually includes one or `more amplier tubes, among other elements.

The level of such video signals is subject to change due to 'both the scanning tube itself and changes in the voltage applied to operate the scanning tube and other tubes oi' the system. This invention relates to a system'by which the output signal level can be maintained at substantially constant level through appropriately controlled operating circuits, known generally as automatic volume control elements'.

In the prior art, some ycircuits have heretofore been provided for establishing an automatic volume control signal to compensate' for changes in output signal level from the scanning tube. However, the circuits of the prior art do not satisfactorily provide an adequate control due to the fact that video, shading, dark-spot correction and many other forms of signals are frequently de veloped during the blanking periods of the scanning cycle in the'television system, it being, of course, understood that this invention is intended for use with a storage type of scanning or camera tube, as above suggested and indicated. Such prior art systems tended to. establish any control 2 but rather by a mixture of it with a plurality of undesired or compensating signals.

It is, therefore, an object of the present invention to provide an automatic volume control circuit for television apparatus which will operate to control output signal level only during periods of luseful video signal transmission, but which,

cuit where the control operates substantially only during the period of useful video signal transmission and is effective upon the video signal transmission rather than upon other signals.

Another object of the invention is that of providing an automatic volume control system for television which operates substantially only duringthe periods between successive blanking signal energy pulses, where the blanking signals are caused, as known in the art, to follow each line of video signal transmission.

Still other objects of the invention are those of providing a sensitive automatic volume con-v of the signal level in such a way that the resulting Y control voltage or energy was not always a signal which was measured by the video signal itself,

trol which iscontrolled positively in its cycle or period oi operation and which, when so controlled,

will respond to producing an emcient and accurate control signal by means of a relatively simple circuit arrangement in which the use of excess parts and weight is avoided.

Other objects of the invention are those of overcoming one or more known defects of the prior art systems of control by. means of an efficient and inexpensive circuit addition which is both positive and sensitive in its operation.

Still other objects and advantages of the invention will become apparent from the specification 'to follow when the specication is read Ain connection with the accompanying drawing, of which the single figure thereof illustrates one suitable form of circuit arrangement by whic the invention may be practiced.

Referring now to the drawing, itwill be seen that the invention essentially comprises the use of a diode tube which derives energy from the output of a suitable video amplier or clipper tubel unit, with provisions having been made to introduce the blanking signal-energy upon the diode element in such a manner as to key it to an inoperative state during the periods of and for the duration of the blanking signal energy pulses.

-ing one form of device, be considered as one of the type known in the art as the Iconoscope" which has-been describedwidely in the literature by Zworykin, Iams, Rose and others, as, for instance, the book above mentioned, and thus requites little or no further reference herein as to its specific properties or construction.

For developing the television video or picture signals, the optical image of the scene I1 to be transmitted is focused upon the mosaic element II of the camera tube I5 by means of a suitable optical system I9, conventionally illustrated. When this is done, electrostatic charges of magnitudes proportional to the intensity of light and shadow of the optical image are built up across the mosaic element. In order to release these charges to produce the output video signals, a scanning cathode ray beam 2| is arranged to scan the mosaic in a suitable pattern under the influence of a suitable defiecting field. The scanning is carried forward in two mutually perpendicular directions by means of suitable deflecting elements 23 and 24, which are herein illustrated as electromagnetic coils, but which could be electrostatic defiecting plates, or they could be electromagnetic coils -for one direction of deflection, and electrostatic defiecting plates for the other direction of deection.

The video signals which are thus produced by the scanning beam action serving to neutralize 4 Where the amplifier units are so set up that the D. C. correction will not again be "lost" (due, for example, to the so-called ALC. coupling circuits), the D. C. component may be added in.

the combining amplifier unit 29 as a so-called D. C. correction signal. Similarly, the blanking and` synchronizing (also called the sync) signals may be added inthe combining amplifier unit.' These signals are, respectively, to suppress the scanning. cathoderay beam of a cathode ray image reproducing tube during' the beam snap-back or return-trace, and for theV purpose ci. maintaining all receiver units in synchronous operation with the transmitter when the video the stored charges on the mosaic in sequence Y are then amplified, first in an amplier 25 which has generally become known as the pre-amplifier since it is located closely adjacent the scanning tube and precedes the general main amplifier unit 21 to which its output signals are supplied. The, amplifier 25 is of a purely conventional type and it, like the other amplifiers to be mentioned and diagrammatically illustrated, has been so widely described in the published literature as to require neither detailed explanation of its circuit arrangement nor its specific manner of functioning.

As was above stated, the signal output from the preamplier 25 is supplied to another amplifier unit 21 which usually comprises several stages of amplification and is frequently termed the line amplier or voltage amplifier." It will be in one of the stages of this line amplifier unit 21 that the control signal energy to be developed by the circuit elements herein to be described is introduced to control the resulting output'signal energy level.

Output energy from the line amplifier 21 is then supplied to a combining amplifier unit 29. In this last named amplifier unit (which also includes one or more tubes under most conditions), it is customary to introduce an appropriate signal to care for and compensate, as far as possible, for a distortion in the video signal which is known in the art as the dark spot. A signal of this compensating character is lintroduced to compensate for the above named effect because of the presence of such distortion due to an .inherent property of the assumed type of scanningtube. The correcting signal thus becomes known as the shading control signal" or the "dark spot correction signal and may be of the form already explained in the literature.

and control signals are received.

Output signal energy from the combining arnplifier unit 29 is nthen supplied by conductor 3l to the control electrode 39 of the video amplifier or clipper tube. 35 to control the signal energy flow therethrough. Output signal energy (representing. the combined signal) is then derived from the amplifier 35 by way of its cathode 31 and the cathode load resistor 4I which connects at one of its terminals to ground at 39. The current flowing through the cathode output resistor produces the voltage drop which is transferred at the output terminal 43 to a suitable load circuit or utilization apparatus (not shown).

Operating voltages for the tube 35 are provided by way of an energy source (not shown) having its positive terminal connected at terminal 48 and arranged yto supply operating voltage to the plate or anode 35 of the tube 35 by way of the peaking inductance element 41 and the plate load resistor 45. Suitable bias is applied to the control electrode 33 of the video am- Y plifier 35 by way of the conventionally indicated bias resistor 40. The bias or clipping level on tube 35 is set automatically by the resistor 40. for instance.

An automatic volume control diode tube 59 is connected so that itspiate or anode 51 connects by way of the coupling condenser 53 and conductor 5I to the junction point 50 of theplate istic of tube 35. The anode 51 of the diode 59v is likewise biased to ground by way of resistor 55. In the arrangement disclosed, it will be apparent that with the signals for operating the tube 59 being. derived from the inductance or choke element 41, only the higher frequency video signals wlllecome effective at the anode 51 of this .dio e To establish a control cycle for the operation of the diode element 59, in order that the diode may be restricted in its operative periods `to those when useful video signals are to be derived from the output terminals 45, the cathode 5I of the diode element 59 connects to ground 39 by way of 'the cathode resistor 51. and blanking signals of positive polarity are supplied at the terminal input 33 and fed by way of the coupling condenser 55 tothe diode cathode 3l across the resistor 51. In this way, the cathode element 5I of the diode 59 is carried highly positive relative to its anode or plate 51 each time a blanking signal is impressed at the input terminal 53. Under these conditions, the diode element 59 ceases to draw current and it becomes inoperative as an automatic volume control tube during the periods when blanking signals are supplied. Otherwise, a control voltage is developed to become effective in the conductor 59 which connects the diode 5I diode 59 back to the ampliiier 21, there is 121-.

eluded a illter combination comprising the `resistor ll and the` capacitor 13 shunting it to ground 39. This lter is so designed and operated that any video or picture signal current which might iowin the conductor 69 is ltered out, and the voltage which reaches the amplifier 21 to lcontrol it is a measure of the effect of the diode 59 to establish a predetermined signal level output from the amplifier 21. Resistor 13 is a.

protective resistor for usual purposes. While not illustrated, it will be appreciated that the diode voltage which is effective on the tubes of amplier 21 is applied in 'such la known manner as to bias one or more of the control electrodes to such a value that the output signal level is established by the diode current.

In the operation of the system described, it will be appreciated also that if the diode tube 59 is rendered ineffective to produce a control voltage for the ampliiier 21 during the blanking period or interval, and with the lter combination 1l and 13 serving to filter any video 0r picture sig nals from being fed back, the control voltage effective upon the amplier 21 to set its gain level will be that voltage which is a measure oi the signal level variation during the video signalling period and will not be disturbed by conditions which occur to cause undesired signals during blanking intervals.

While the form of invention hereinabove explained illustrates a circuit wherein the voltage for operating the automatic volume control has been derived from the plate or anode circuit of the clipper tube by the connection at point B, a modification might readily be made where the controlling voltage was derived froma similar connection'ma-de to the input circuit of the clipper tube 35. This, for instance. might bev by way of a connection established on 'the conductor 3i and substantially at the grid or control electrode 33 of the clipper tube 35.

It, oi course, will be appreciated, under such circumstances, that the polarity of the signal supplied to the diode 59 must be made to correspond to that with the circuitl diagram shown when the modification above suggested is made so that a relay or reversing tube may frequently be used. Alternatively, where the signal polarity is such as to demand it, the diode connections may be reversed and a negative synchronizing impulse applied at terminal 83.

Having now described the invention what is claimed is:

1. In a television video system kwherein 4produced video -signals are ampliiied and combined for transmission with atleast one control signal occurring at predetermined spaced time intervals.

duce trains of video signals, a source of control signals occurring lor predetermined time periods and with predetermined time spacings therebetween for combination with the produced video signals, a video signal amplier unit, a combining amplier unit for combining and amplifying the video and the control signals, an output amplifier, a rectier element, means for supplying energizing voltages to the rectiiier from the signal output of the output amplier, means for applying a biasvoltage to the video signal ampliiier to control the signal amplification therefrom in accordance with the current flow through the rectier, and means for keying the rectiiler by the control signals to limit the operative periods o! the rectier to time intervals intermediate each control signal.

3. An automatic volume control system for television amplifier units comprising a video signal ampliiler unit, a source of signals regularly occurring at spaced time, intervals for. controlling the video signals, a combining amplifier for combining the amplified video signals and the control signals, means to supply the combined signais to a load circuit, a rectifier element, means. i

to energize the rectifier by the produced cornbined signals whereby the current ow through the'rectifier represents the signal output level of the amplifier unit, means to supply a control voltage to the video signal amplifier which is-determined by the current flowing through the rectitler, and ineans also to supply the control sig--` nais from the source to the rectiiier to control the rectiiler independently of the combined video and control signals in a manner to bias the rectiiler to a cutoi state during periods when the control signals are present.

4. A television video amplifier system wherein produced video signals arev amplified and combined for transmission with at least one control signal occurring at predetermined spaced time in-V v render the rectier inoperative to develop the a signal output control comprising an amplifier Vfor combining and amplifying'the produced sig- -trol of the current flowing through the rectiiler. and means to render the rectiiier inoperative to apply `the said bias voltage during the periods where the control signals are combined with the video signal.

said bias voltage during time periods where the control signals are combined with the video signal.

5. A television system comprising means to produoe trains of video signals, a source of control signals occurring for predetermined time periods and with predetermined long time spacings therebetween as compared with the signal duration for combination with the produced video signals, a video signal amplifier unit, a combining amplier unit for combining and amplifying the video and the control signals, a diode rectiiier element,V

means for supplying energizing voltages lto the said diode rectiiier from the signal output of the combining ampli-iler, means for developing a bias voltage to he applied to the video signal ampliiier to control the signal amplication therein with the bias voltage being developed in accordance with the current tlow through the said diode rectiiler, and means for keying the saiddiode rectifier by the control signals to limit the operative periods of the rectier to time intervals in- 2. A television'system comprising means to' pro- 'u termediate each control signal.

` tive to the video signals, a combining means for combining the said video signals and the control signals into a combined video and control signal wave train.- means to supply the combinedsignals to a load circuit, a rectiiier element, means to supply the combined signals to the rectifier to control the current flow therethrough in accordance with the signal level of the combined signals so as to develop a control voltage proportional to the current flow, means to supply the control voltage to control the signal level of the signalssupplied to the load circuit, and means also t supply the Aregularly occurring control signals to the rectier to control the rectifier independently of the combined video and control signals in a manner to bias the said rectiiier to a cutoii state during periods when the control signals are present.

7. In a television video system wherein produced video signals are amplied and combined for transmission with at least one control signal occurring at predetermined spaced time intervals, a signal output control comprising an amplifier for combining and amplifying the produced signals, a rectifier element connected to receive signal energy output from the ampliner and normally to draw current which is a measure of the average signal output level, means foxI applying a bias voltage to the amplifier underthe control of the current flowing through the rectifier, and means to supply the control signals to the rectifier independently of the combined signals to render the rectifier inoperative to apply the said bias voltage during the periods when the control signals are combined with the video signals.

8. A television system comprising means to produce trains of video signals. a source of control signals occurring for predetermined time periods and with predetermined long time spacings therebetween as compared with the signal duration for combination with the produced video signals, a video signal amplifier unit, a combining amplifier unit for combining and amplifying the video and the control signals, a diode element having its anode electrode element connected to receive combined Vvideo and control signal output energy, means for developing a bias voltage for the video i signal amplifier to control the signal amplification therein invaccordance with the current flow WALDEMAR J. PocH. 

